Model Output Soundings and Meteograms
Scientific Services Division
MODEL OUTPUT SOUNDINGS. Not that many years ago forecasters were limited to viewing model output as plan views at 12 hour intervals. Once gridded model output became available, it was possible to also display the output as cross sections and local time sections. However, the spatial and temporal resolution of the cross sections and time sections, even on AWIPS, has been limited by the resolution of the model output distribution grids.
Several years ago, NCEP began producing forecast model output soundings for selected sites. Stored in the BUFR format, these soundings are made for every hour during the model forecast period using the full spatial resolution of the numerical model. (Recall that the model integration time steps are on the order of minutes.) Model output soundings from the NGM, Eta-32 and RUC-2 are available on the OSO server. Soundings from the AVN are expected to become available later this year after the model is migrated to the new Class VIII computer. The NGM and Eta-32 soundings are stored in individual files for each site. Due to processor limitations on the NCEP Cray computer, the soundings for the RUC-2 are currently stored in a single, large file for all stations.
Forecast model soundings are also available at several NOAA and university sites on the Internet. Links are available through SSD's Numerical Weather Prediction Web page: http://www.srh.noaa.gov/ssd/nwpmodel/html/nwplinks.htm
Keith Brill at NCEP developed a UNIX/LINUX script, called modsnd, that downloads the model soundings and converts them from BUFR to the GEMPAK -- and optionally, to the BUFKIT -- format. (The BUFKIT PC program is a forecast profile visualization and analysis tool kit developed by the staff at NWSO Buffalo, NY. The program is available in the /ext1/download subdirectory on the Southern Region model output server, and at http://www.wbuf.noaa.gov/bufkit/.) A companion script to modsnd creates graphical metafiles of the soundings for display with NAWIPS, or as GIF files for posting on a Web server.
METEOGRAMS. In addition to the forecast model soundings, NCEP also outputs forecast surface files from which meteograms can be made. Figure 1 is a sample meteogram from the Eta model for Dallas-Fort Worth as displayed in NAWIPS. Note that it includes variables, such as percent of low, middle and high clouds, which are not explicitly computed by the model. These are parameterized from the model variables.
Low clouds are assumed to occur in the model layers from the surface to 642 mb; middle clouds, from 642 to 350 mb; and high clouds, from 350 mb to 50 mb. The grid level with the maximum relative humidity in each layer is used to specify the cloud fraction in the layer.
The percent of cloudiness for any single grid point is estimated from the forecast relative humidity:
cloud fraction = 1 - ( (100% - RHfcst) / (100% - RHcrit) )½ for RHfcst > Rhcrit
cloud fraction = 0 for RHfcst # Rhcrit
where Rhfcst is the forecast relative humidity, and Rhcrit is the "critical" value (75% over land and 80% over water) at which clouds are parameterized to begin forming.
Figure 2 is a graph of the cloud fraction vs. relative humidity relationship.
Forecast meteograms are available at several NOAA and university sites on the Internet. Links are available through SSD's Numerical Weather Prediction Web page: http://www.srh.noaa.gov/ssd/nwpmodel/html/nwplinks.htm. Until recently, NCEP made available meteograms for only 88 sites via the Web (http://ftp.ncep.noaa.gov:8000/research/meteo2.html). The NCEP recently added the display of low, middle and high clouds and sensible weather to their surface meteograms, increased the number of viewable sites four-fold, and added sites in Alaska, Hawaii and Puerto Rico.
INCREASED NCEP SUPPORT FOR TERMINAL FORECASTS. As reported previously (Technical Attachment 99-11) NCEP expects to increase their support for TAF forecasts later this month by producing model forecast soundings, meteograms and time sections for every TAF site in the North American domain of the Eta-32 from all four runs of the model. This will occur once the Eta-32 model is moved onto the new Class VIII computer.
The generation of the BUFR sounding files will be elevated to full operational status within NCEP's production suite which will maximize the reliability of the model forecast soundings. Cloud water and ice, cloud-base pressure, and low-level visibility will be added to the soundings. The NCEP is working with OM and OSO to establish a WMO labeling capability for the BUFR soundings to allow them to be sent through OSO to the AWIPS Satellite Broadcast Network (SBN) for official distribution. Eventually, everyone (in NWS) will be getting these data via SBN and manipulating and displaying the guidance information with AWIPS.
In the interim, NCEP is developing a Web site to display surface meteograms and time sections for all the TAF sites until the model sounding data are placed on the SBN. A sample of a time-height display under consideration can be viewed at: http://ftp.ncep.noaa.gov:8000/research/meteo1.html. The expanded BUFR site list, cross-referenced with the current list and site maps is available at: http://sgi62.wwb.noaa.gov:8080/BUFR_TAF/index.html.
To preserve bandwidth on our dedicated data line to NWSH we plan to make the model soundings for all Southern Region TAF sites available on our regional model output server. We will distribute a version of the updated modsnd scripts that will first look for the files on our server before searching the OSO and NIC servers.